全文获取类型
收费全文 | 7073篇 |
免费 | 8篇 |
国内免费 | 34篇 |
专业分类
航空 | 3629篇 |
航天技术 | 2561篇 |
综合类 | 32篇 |
航天 | 893篇 |
出版年
2021年 | 40篇 |
2019年 | 46篇 |
2018年 | 70篇 |
2014年 | 115篇 |
2013年 | 166篇 |
2012年 | 142篇 |
2011年 | 207篇 |
2010年 | 150篇 |
2009年 | 240篇 |
2008年 | 323篇 |
2007年 | 164篇 |
2006年 | 179篇 |
2005年 | 185篇 |
2004年 | 165篇 |
2003年 | 221篇 |
2002年 | 138篇 |
2001年 | 225篇 |
2000年 | 138篇 |
1999年 | 182篇 |
1998年 | 219篇 |
1997年 | 140篇 |
1996年 | 203篇 |
1995年 | 257篇 |
1994年 | 238篇 |
1993年 | 148篇 |
1992年 | 172篇 |
1991年 | 91篇 |
1990年 | 84篇 |
1989年 | 192篇 |
1988年 | 78篇 |
1987年 | 85篇 |
1986年 | 81篇 |
1985年 | 259篇 |
1984年 | 195篇 |
1983年 | 167篇 |
1982年 | 171篇 |
1981年 | 239篇 |
1980年 | 83篇 |
1979年 | 62篇 |
1978年 | 67篇 |
1977年 | 66篇 |
1976年 | 47篇 |
1975年 | 81篇 |
1974年 | 51篇 |
1973年 | 60篇 |
1972年 | 66篇 |
1971年 | 54篇 |
1970年 | 58篇 |
1969年 | 55篇 |
1967年 | 38篇 |
排序方式: 共有7115条查询结果,搜索用时 327 毫秒
991.
R.J. Bayuzick N.D. Evans W.F. Hofmeister K.R. Johnson M.B. Robinson 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(5):85-90
The 100 meter high drop tube at the Marshall Space Flight Center has proven to be a viable facility for studies of containerless solidification. Advantages are that experiments are inexpensive and large numbers of specimens can be processed rapidly. It would not be unusual to run ten specimens in a day. Another significant advantage is that the undercooling behavior can be followed with sufficient sensitivity to easily detect the onset of recalescence and subsequent events.Disadvantages are the restrictions on specimen sizes and types of alloys that can be run in a microgravity environment. Practical specimen sizes range between 50 mg and 500 mg depending on the type of furnace being used. Refractory alloys can be processed in a vacuum (about 10?5 torr) and therefore at microgravity. Non-refractory alloys demand an atmosphere (about 200 torr) to obtain appreciable undercooling before impact at the bottom of the tube. Under these conditions significant g forces result.Because of the present limitations of the 100 meter drop tube, the most definitive work has been done on niobium based alloys. Large amounts of undercooling have been observed routinely and the effects of undercooling on microstructure have been characterized in detail. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have been used to determine types of phases, amounts of phases, and compositions of phases. It is clear, as would be expected, that the results bear some resemblance to rapid solidification processing by quenching. However, there are dissimilarities due to the uniqueness of solidification by deep undercooling without quenching in long drop tubes and accompanying recalescence effects. 相似文献
992.
Theoretical and experimental aspects of satellite aerodynamics in free molecular flow have been studied. A FORTRAN-program package has been developed to calculate free molecular aerodynamic force and moment coefficients of general and special convex shapes, using an approximation of the surfaces by plane triangular elements.As a special case for concave bodies, the drag of a satellite of TD1A-similar shape has been investigated by an approximate theory and by wind tunnel experiments. Comparisons are made between experimental results, the approximate theory and exact theory. 相似文献
993.
G. Erdós J. Kóta 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(3):139-142
We consider regular motion of 50 – 200 GV particles in a large-scale interplanetary magnetic field model which contains a wavy neutral sheet responsible for the sector-structure. Numerical calculations based upon energy losses along various trajectories are carried out to obtain the predicted omni-directional density and anisotropy of cosmic rays at various solar latitudes. A marked difference is found between odd and even solar cycles. The post-1969 field configuration gives small radial and large latitudinal gradient: cosmic ray density increases toward the poles. The latitudinal gradient turns out smaller and of opposite sense for the pre-1969 epoch. Anisotropy changes dramatically as we move off the solar equator: corotation appears to be restricted to low latitudes. 相似文献
994.
995.
Mclane P.J. Campbell L.L. Mesiya M.F. 《IEEE transactions on aerospace and electronic systems》1976,(4):519-522
The loss in output signal-to-noise ratio (SNR) due to amplitude limiting is obtained for a radar circuit consisting of a bandpass limiter, coherent demodulator, matched filter, and moving-target-indicator (MTI) filter. The circuit is used in scanning MTI radars. The tandem connection of the limiter and coherent demodulator is a model for the saturation of the intermediate-frequency (IF) demodulator of an MTI radar. Results on special functions are used to obtain simple formulas for the loss in output SNR relative to a linear IF demodulator when the input SNR is less than -15 dB and the number of hits per 3-dB beamwidth exceeds 15. 相似文献
996.
997.
M.J. Rycroft P.S. Cannon T. Turunen 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(2):449-454
ELF and VLF radio signals were recorded in the afternoon to early morning (local time) between 24 March and 4 April 1979, in Northern Scandinavia. Apart from signals of natural origin, timing signals, i.e. six pips of equal duration of 105 ± 8 ms, at 1 kHz ± 0.5 Hz, were observed on the hour UT. Such signals only occur on days of relatively high geomagnetic activity during enhanced auroral electrojet activity. They are believed to be generated by non-linear demodulation (self-detection) of signals from two or more amplitude modulated transmitters in the USSR, operating at 173, 200, 236, 263 and 657 kHz. The simplest explanation for the observations is provided by the three transmitters operating at 173 kHz. 相似文献
998.
H. Oya T. Ono 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(2):217-220
The stimulated plasma wave experiment (SPW) has been successfully carried out in the plasmasphere and the magnetosphere along the JIKIKEN (EXOS-B) satellite orbit where the plasma parameters indicate wide variety of the combination of the electron number density, ranging from 1/cc to 104/cc, and the electron cyclotron frequency, ranging from 6 kHz to 200 kHz.The upper hybrid resonances FUHR usually persists for long periods up to 125 msec and the electron cyclotron resonances nFH are stimulated at frequencies with the very high harmonic number n; sometimes, the nFH resonance takes place for n=47.All the features of the resonances including FOn reflect the characteristics of the magnetospheric plasma that contains the energetic and non-Maxwellian components of the particles. The measurement of the plasma resonance contributes to the detection of the local electron density and the magnetic field intensity. The mode of the propagating radio waves is also determined being compared with the observed local plasma resonance frequency Fp. 相似文献
999.
A Chatterjee P Koehl J L Magee 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1986,6(11):97-105
A theoretical approach to the understanding of the biochemical mechanisms of indirect action of ionizing radiation on SV40 DNA in aqueous solution is presented. The extent of OH attack on the sugar moiety and bases has been calculated. A realistic model for the DNA (in B form) based on available X-ray diffraction data is used and specific reaction sites for the OH radicals are obtained. A Monte Carlo scheme is used to follow the diffusion and reaction of the OH radicals. Effects of track structure have been considered and the single strand break D37 values for 14 MeV electrons (low-LET) and 670 MeV/u and 40 MeV/u neon particles are presented. Calculated results are in agreement with available experimental data. It has been found that regardless of the qualities of radiation, 80% of the OH attack on DNA is on the bases and 20% is on the deoxyribose. From probability considerations only, it appears that the number of double strand breaks varies linearly with dose. 相似文献
1000.
J. Perez-Peraza 《Space Science Reviews》1986,44(1-2):91-138